In order to measure the pressure of a fluid flowing in a pipe, it is known to use a measurement device comprising a coupling for fixing in a tapping hole in the pipe and a ceramic-diaphragm pressure sensor fixed to the coupling. The ceramic diaphragm has resistor tracks on one face that are connected as a Wheatstone bridge, as shown in FIG. 7, and the deformations of the diaphragm under the effect of pressure unbalance the bridge, with said unbalance being detected by electronic means that are suitable for generating a signal that is representative of the pressure.
Proposals have been made, in particular in the food industry, to use pressure-measuring devices in which the pressure sensor is isolated from the fluid flowing in the pipe by a stainless steel diaphragm placed so as to be flush with the inside surface of the pipe. The space between the metal diaphragm and the ceramic diaphragm of the sensor is filled with a liquid such as oil that is suitable for transmitting pressure. That solution does not provide entire satisfaction since the metal diaphragm can break and allow the pressure-transmitting liquid to escape, thereby contaminating the fluid flowing in the pipe. Furthermore, the pressure sensor can continue to operate without it being immediately apparent that the metal diaphragm has broken, since the fluid flowing in the pipe continues to exert pressure on the ceramic diaphragm. Finally, the space between the metal diaphragm and the ceramic diaphragm is relatively protected from the flow of fluid in the pipe, and that can encourage the proliferation of bacteria, and even when detected quickly that means that production must be stopped until the origin of the problem has been found, and all of the products made during the period believed to be suspect must be rejected.
U.S. Pat. No. 5,869,766 discloses a pressure-measurement device comprising a removable isolating disk of polymer interposed between the pressure sensor and the fluid flowing in the pipe. The coupling is preferably made of the same polymer as said disk. The coupling rests against a step formed in the wall of the pipe. That measurement device presents the advantage of avoiding the use of a pressure-transmitting liquid and eliminates the risks associated therewith, but it presents the drawbacks of creating a dead space in front of the isolating disk, because of the presence of the above-mentioned step, and also of being unsuitable for measuring negative pressure. Although the existence of dead space is considered as a drawback (column 10, lines 23-28), no solution is proposed for remedying that drawback, so that measurement device is ill-adapted to the food industry.